Strains of yeast for bread-making and novel strains of yeast thus prepared

ABSTRACT

A complete and reproducible process for producing novel strains of yeast, comprises making a first screening test and at least one other screening test selected from a group of screening tests which do not resort to any measurement of gas release, selecting by means of said first and at least one other said screening test the desired strains from a group of diploid strains prepared previously either by hybridation, or by mutation of existing strains. The tests are as follows: A first test consists of measuring the average multiplication coefficient of a given strain by following the optical density variation of a standard medium seeded by a suspension of cells obtained from this strain. A second test consists of measuring in the same manner the average multiplication coefficient of the said strain in the presence of an inhibitor acid added to the standard medium. A third test consists of measuring the maltose adaptation of said strain in the presence of glucose by determining the amount of maltose subsisting in a standard medium after a known amount of glucose added to this medium has been completely consumed. A fourth test consists of measuring the invertase content of said strain. A fifth test consists of measuring the latent time of said strain. The hybridation can consist of systematic haploid crossings derived from quick Saccharomyces Cerevisiae strains adapted to maltose and haploids derived from very slow strains not adapted to maltose, but well adapted to sweet doughs and sometimes also to acid doughs.

This application is a continuation-in-part of earlier filed co-pending application Ser. No. 931,151, filed Aug. 4, 1978, now U.S. Pat. No. 4,318,930 which is in turn a continuation-in-part of prior co-pending application Ser. No. 863,953, filed Dec. 23, 1977 (now U.S. Pat. No. 4,318,929).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a complete and reproducible process for obtaining new strains of yeasts for bread making. It also relates to the novel strains of yeast thus prepared as well as to fresh or dried yeast for bread making prepared from said new strains.

2. Description of the Prior Art

Quick strains of yeasts are already known which are adapted to maltose--that is to say, enabling the preparation of yeasts which release a large amount of CO₂ with doughs constituted by flour and water--and which remain active with doughs of little sweetness, that is to say, not containing more than 5% by weight of sugar with respect to the flour, namely less than 3.3% by weight with respect to the dough.

It is found that these strains exhibit performances which drop appreciably when the sugar content of the dough increases and notably when it exceeds 10% by weight with respect to the flour. Now, such doughs represent a non-negligeable part of bread-making in certain countries.

Moreover, these strains, whose use has been generalised in yeast making for about 10 years, are rapdily inhibited as soon as the dough contains significant concentrations of acetic, sorbic or propionic acids or their salts.

Acid or sour doughs occur in the manufacture of rye bread, leavened bread and others, whose acidity, corresponding to a pH below 4.7, is contributed by a mixture of about 10-50% by weight of acetic acid and about 50 to 90% by weight of lactic acid and which are rarely sweetened, represent also a non-negligeable part of bread making.

Finally, in all countries, there is added to the products of bread making intended to have a long period of preservation or a preservation under difficult conditions, mould-inhibiting agents such as acetic, sorbic, propionic acids and their salts, and this whatever the proportion of sugar contained in the dough. It is known that a bread making yeast resistant to acetic acid, that is to say, whose fermenting power is not inhibited significantly in the presence of undissociated acetic acid, has generally the same property, that is to say, a better resistance, with respect to inhibiting doses of undissociated propionic or sorbic acid.

To overcome the inadequacies of the prior art with regard to bread-making yeast strains, it is an object of the invention to provide a process adapted to permit the obtaining, in simple and reproducible manner, of novel strains of yeast adapted to maltose, characterised by the fact that the yeast, both fresh and dried, of which they enable the preparation and of which certain at least constitute novel industrial products, are:

either still better adapted to maltose,

or active with sweetened doughs, that is to say with dough containing at least 5% by weight of sugar with respect to the flour,

or active with acid doughs,

or endowed, preferably, with two of the three above-mentioned properties and preferably with all three.

In order to do this, the specialist in the field has the choice between:

on the one hand, modifying the processes of propagation of yeast, that is to say, its processes of cultivation and,

on the other hand, obtaining novel strains by mutation and/or hybridation.

Modification of the cultivation processes is laborious and difficult to put into practice and often magnifies one property more or less to the detriment of another.

Research for novel strains by hybridation and mutation poses complex problems. It is highly uncertain if the objectives and the phenomena in play are not well mastered and if the crossing plans or the mutation process are not clearly defined. It results in any case in the obligation to test thousands even tens of thousands of colonies, which is impossible in practice by means of tests with release of gas under specific conditions (flour, sugar or organic acid mediums), said tests requiring cultivation in a fermenter of some liters as described in Example 1 of French patent application No. 75 20943, the harvesting of this yeast and at least five measurements of gas release according to tests of type A which will be considered below. The problem is complicated by the fact that the results obtained are reproducible with difficulty; for example, a slight modification, difficult to master in the conditions of cultivation can result in considerable variations in respect to the criteria measured.

Nevertheless, research for novel strains is theoretically the best solution, all the more as the employment of specific cultivation conditions can only improve, reinforce the natural properties possessed by the hybrids or mutants.

The two routes of research are in fact complementary and not concurrent.

GENERAL DESCRIPTION OF THE INVENTION

The process according to the invention is characterised in that by means of the first and of at least one other screening test selected from a group of screening tests not calling upon any measurement of gaseous release, the desired strains are selected from a group of diploid strains previously prepared either by hybridation, or by mutation of the existing strains, the tests of said group of tests being constituted by:

a first test consisting of measuring the mean or average multiplication coefficient of a given strain by following the variation of the optical density of a standard medium seeded by a suspension of cells obtained from this strain,

a second test consisting of measuring in the same manner the means multiplication coefficient of said strain in the presence of an inhibitor acid added to the standard medium,

a third test consisting of measuring the maltose adaptation of said strain in the presence of glucose by the determination of the amount of maltose subsisting in a standard medium after a known amount of glucose added to this medium has been completely consumed,

a fourth test consisting of measuring the invertase content of said strain, the invertase unit being defined as production of a micromole of reducing sugars in five minutes per mg of yeast dry matter at 30° C. and at pH 4.7, without plasmolysis of the yeast, namely a demi-micromole of invert saccharose,

a fifth test consisting of measuring the latent time of said strain, that is to say the delay in the starting of multiplication, by following the variation in the optical density of the yeast suspension applied in the first test after having conferred on this suspension a sugar concentration of at least 20%.

In the abovesaid process, for the first test, it is possible to produce a preculture of 48 hours at 30° C. in a test tube containing 10 ml of a medium having the following composition:

    ______________________________________                                                yeast extract                                                                           1%                                                                    peptone  2%                                                                    saccharose                                                                              2%                                                             ______________________________________                                    

denoted in the following description by the name "YEP medium".

The preculture can also be made in other media.

A known amount of this preculture serves then for seeding an optical flask containing 30 ml of a medium identical with or equivalent to the YEP medium. The growth curve of the yeast is then established by measurement of the optical density every hour at 600 mμ.

From this curve, it is possible to calculate the mean multiplication coefficient μ given by: ##EQU1## where x represent the cellular population at the time t.

This first test has a double advantage in the sense that it can serve as a control for the other tests and that it permits the elimination of all the diploid strains not having a sufficient multiplication coefficient, that is to say, capable of giving in sufficient yield or of having too low a gaseous release, as for example strains which have been treated too severely during a mutation treatment.

Again in the above-mentioned process, for the second test, the inhibitor acid added to the cultivation medium consists of acetic acid or of a mixture of acetic acid and lactic acid in an amount and in proportions such that it inhibits to a ratio comprised between 50% and 90% the growth of a control strain showing on dough containing inhibiting organic acids and/or on sweetened dough, a considerable reduction in its power of CO₂ release.

It has, in fact, been found that there existed a non-negligeable probability that the strains which, in a medium containing an amount of acetic acid inhibiting in a large proportion the growth of known quick yeast hybrids adapted to maltose, show a better growth than these hybrids, and exhibit a least fermentation inhibition with acid doughs (more than 50% of chances), or, and are more active and often distinctly more active with sweetened doughs than said quick hybrids, adapted to maltose.

Again in the abovementioned process for the third test, the yeast strain is placed in the presence of a sugar solution containing a known amount of glucose and of maltose. The amount of glucose is selected so that it is entirely consumed at the end of the test which lasts generally one hour. At the end of the test, the reaction is stopped by cold centrifugation and the sugar remaining in the supernatant liquid is determined. The percentage of maltose consumed by the yeast is deduced therefrom. It was found that the percentage of maltose consumed generally well manifested the more or less good adaptation of the tested strain to the fermentation of maltose and also the speed of the strain.

A modification consists of following as a function of time the disappearance of the glucose and of the maltose added together. The disappearance of the total sugars is followed by determination with anthrone and that of the glucose by specific enzymatic determination, the amount of maltose being determined by difference.

It was observed that the strains not adapted to maltose did not ferment, in this test, the maltose as long as glucose remained; on the other hand, the strains adapted to maltose ferment more or less rapidly the maltose even when there is still glucose.

The abovesaid test is carried out by starting with a known amount of dry material obtained from the yeast strain tested. This known amount of dry yeast material can for example be obtained by filtration of the yeast present in the optical flask at the end of the first test, or by filtration of an amount of yeast obtained within the scope of a similar shaken culture and by determination of the dry material in this cake, which dry material is generally of the order of 20%.

The adaptation to the maltose can also be estimated semi-quantitatively by recovering colonies of tested strains spaced regularly on a petri dish by a filter paper soaked in a maltose solution, for example 0.1 Molar and in Bromo-Cresol Purple acidity type indicator dye and by monitoring the colorimetric change at the end of a given time. This coarse test is a first possible approach, essentially after mutation. In any case, the methods described above must always be employed to refine this first possible selection.

Again in the above mentioned process there is indicated, in connection with the fourth test, that the invertase unit may be determined for example in the following manner. It starts with a known amount of dry yeast material of the order of 0.1 to 0.4 mg which can be obtained by filtration of the yeast present in the optical flask at the end of the first test. This amount of dry yeast material is placed in the presence of saccharose at a final 0.1 Molar concentration in a test tube in a buffered medium, buffer acetate at pH 4.7 placed in a water bath at 30° C. At the end of five minutes, the inversion reaction of the saccharose is blocked by the addition of a reactant sodium dinitrosalycilate which serves to determine the reducing sugars formed, by colorimetric reaction.

The greater or lesser richness in invertase can also be evaluated semi-quantitatively by colonies of strains tested on a petri dish, by covering the dish with a filter paper soaked in saccharose and the necessary reactant for the enzymatic determination of the glucose formed, by colorimetric reaction (0 - dianisidine, for example). This test is a first possible approach, essentially after mutation. It can, in some cases, permit a first very rough selection. In any case, the simple and practical method described above must always be employed to refine this first selection.

It has been found that a strong probability exists that all strains of yeast having less than 35 invertase units, preferably less than 30 units and, more preferably still, less than 20 invertase units, are active on sweetened doughs especially if they have given good results in the first test and in the third test.

By way of indication it is to be recalled that the hybrids of quick yeast adapted to maltose, employed until now in yeast making, have an invertase content which is always greater than 40.

Again, in the above mentioned process, it is pointed out, with respect to the fifth test, that the shorter the latent time, the more likely the tested strain is to be osmotolerant, that is to say, active with sweetened doughs.

In conclusion, it is emphasized that the conjugate use of the first and third tests can either permit the selection of strains which are even quicker and better adapted to maltose than those which have been used hitherto, or permit the selection, when it is coupled with at least one of the other tests described, of strains adapted to maltose and more active with sweetened doughs or, preferably and, with acid doughs.

Thus, it has been found that a selection effected by using in conjugated manner, the first screening test (the criterion taken being that any strain selected should have a multiplication coefficient equivalent to that of the best commercial baker's yeast), the third test (the criterion taken being that any strain selected should have an adaptation to maltose corresponding to an amount of maltose consumed equal to at least 50% of the maltose consumed in the operational method taken with the best strains of commercial yeast adapted to maltose) and the fourth test (the criterion taken being that any strain selected should have an invertase content less than 35 units, preferably less than 30 units, and more preferably still, less than 20 units), leads to strains better adapted to maltose and/or more active with sweetened doughs.

All the strains thus selected have novel characteristics with respect to all previously known strains.

It has also been found that a selection effected by using in conjugated manner the first screening test (the criterion taken being that any strain selected should have a multiplication coefficient equivalent to that of the best strains of commercial baker's yeast), the third test, (the criterion taken being that any strain selected should have an adaptation to maltose corresponding to an amount of maltose consumed equal to at least 50% of the maltose consumed by the best strains adapted to maltose) and the second test (criterion taken being that any strain selected should have less inhibition of the culture in the presence of acetic acid added to the standard medium), leads to strains better adapted to maltose and/or more active with acid doughs and/or often equally more active also with sweetened doughs.

All the strains thus selected have novel characteristics with respect to all previously known strains.

A selection effect by means of a combination of the four first tests described above leads to strains having at least two and mostly three of the following properties:

improved adaptation to maltose,

better activity with sweetened doughs having from 1 to 20% of sugar,

better activity with doughs containing undissociated acetic acid.

All the strains thus selected have novel characteristics with respect to all previously known strains.

The fifth test (the selection criterion taken being the search for as short as possible a latent time when there is added to the standard medium of the first test an amount corresponding to a concentration of saccharose in the medium of 30%) can serve to confirm the significance of the results of the fourth test (low invertase content). It can also serve, associated with the first test, and preferably also with the fourth test (the selection criterion taken being an invertase content less than 20 units) for seeking strains particularly active with doughs with 10-25% of saccharose.

Accordingly and still in the above mentioned process, the hydridation according to the invention consists essentially of systematic crossings of haploids derived from strains of quick saccharomyces cerevisiae adapted to maltose and haploids derived from very slow strains, not adapted to maltose, but well adapted to sweetened doughs and sometimes also to acid doughs belonging to the Saccharomyces genus, and notably to the Saccharomyces cerevisiae species.

As indicated above, these strains of quick saccharomyces cerevisiae baker's yeasts adapted to maltose, are well known and are generally those which serve at present for the manufacture of fresh yeast marketed throughout the world. Such yeast are for example described in British Pat. Nos. 868 621 (strain ATCC 13 601), 868633 (strain ATCC 13 602), 989247 (strains CBS Ng 740 and Ng 1777), etc. This list of patents and of strains deposited at the collection centres such as ATCC (American Type Culture Collection), the CBS (Centraalbureau voor Schimmel Cultures de Baarn) or NCYC (National Collection of Yeast Cultures, Colney Lane, Norwich NR4 7UA, England) is not limiting nor complete.

Slow yeast-making strains Saccharomyces Cerevisiae, other strains of Saccharomyces with an osmophile character, like for example strains of Saccharomyces rosei, Saccharomyces rouxii, are available at the principal collection centres such as the ATCC, CBS or the NCYC.

It has been found that certain slow yeast-making strains, used at other times in Europe, and certain distillery strains, like for example the isolate which is deposited at the NCYC under no. R30 and which is described in French Pat. No. 75 20943, were extremely interesting materials for this crossing operation.

Sporulation of the starting strains, the obtaining of the haploids, and the conjugation of these haploids are carried out according to the techniques described in Chapter 7 of "Sporulation and Hybridization of Yeasts" by R. R. FOWELL in the book "The Yeasts", volume 1, edited by Anthony H. Rose and J. S. Harrison, 1969, Academic Press, London and New York. The hybridation technique adopted between haploids of Saccharomyces Cerevisiae was the mass-mating technique. Micro-manipulation may be the preferred method for crossings in which haploids of other Saccharomyces than Saccharomyces Cerevisiae take part.

In this way the strains are obtained which meet the criteria taken in the first and third tests, as well as the criteria taken in one or several of the three other tests.

It has been verified that the large majority of yeasts obtained from strains selected by means of the group of tests of the process according to the invention had at different successive stages of cultivation, namely at the three liter cultivation stages, and then cultivation in a pilot fermenter, the calculated fermentative properties, these properties being measured by means of tests A (carried out by means of the Burrows and Harrison fermentometer) and tests B (carried out by means of the Chopin zymotachygraph) which are described below.

Again in the above mentioned process, the mutation according to the invention which constitutes a completely novel route for producing industrial strains of bread-making yeast, which route it has been possible to take due to the group of tests of the same process, consists essentially of a mutagenesis carried out on the haploid or on the diploid and applying mutagenic agents such as ethylmethane sulfonate and N-methyl, N-nitro, N-nitrosoguanidine or NTG, the haploids obtained at the end of the mutagenic treatment being then utilisable as crossing material in the hydridation operations.

Preferably, recouse is had to mutagenesis with NTG resulting in a survival rate comprised between 2% and 80%.

The survival rate selected must be higher operating with haploids than when operating with diploids so that these haploids retain their aptitude to be conjugated. The survival rates generally selected for working with haploids have been comprised between 40% and 80%.

This being the case, the novel quick strains, adapted to maltose, obtained by the application of the process according to the invention, may be characterized by the fact that they enable the preparation of both fresh and dried yeast, themselves characterized by their release of gas that they give within the scope of a certain number of tests denoted by the references A (A₁, A'₁, A₂, A'₂, A₃, A'₃, A₄, A'₄, A₅, A'₅) carried out by means of the Burrows and Harrison fermentometer and by the references B (B₁, B'₁ and B'₃) carried out by means of the Chopin zymotachygraph and which will be defined below.

Test A₁ (fresh compressed yeast)

To 20 g of flour incubated at 30° C., is added a weight of compressed yeast corresponding to 160 mg of dry material, this yeast being diluted in 15 ml of water containing 27 g of NaCl per liter and 4 g (NH₄)₂ SO₄ per liter; it is kneaded by means of a spatula for 40 seconds, so as to obtain a dough which is placed on a waterbath regulated to 30° C.; 13 minutes after the beginning of kneading, the vessel containing the dough is hermetically closed; the total amount of gas produced is measured after 60, and then 120 minutes; this amount is expressed in ml at 30° C. and under 760 mm of Hg,

Test A'₁ (dry yeast)

Identical with test A₁, but prior to kneading, the dry yeast is rehydrated in distilled water, at 38° C.; for this purpose 40% of the volume of water of hydration applied is used; the complement of water, supplemented with 405 mg of NaCl, is added at the end of the 15 minutes of rehydration,

Test A₂ (fresh compressed yeast)

Test identical with test A₁, but there is added to the flour 100 mg of saccharose; the total amount of gas produced is measured after 60 minutes,

Test A'₂ (dry yeast)

Test identical with test A'₁, but there is added to the flour 100 mg of saccharose; the total amount of gas produced is measured after 60 minutes,

Test A₃ (fresh compressed yeast)

Test identical with test A₁, but there is added to the flour 2 g of saccharose; the total amount of gas produced is measured after 60 minutes,

Test A'₃ (dry yeast)

Test identical with test A'₁, but there is added to the flour 2 g of saccharose; the total amount of gas produced is measured after 60 minutes.

Test A₄ (fresh compressed yeast)

Test identical with test A₁, but to the flour is added 5.5 g of saccharose; the total amount of gas produced is measured after 60 minutes,

Test A'₄ (dry yeast)

Test identical with test A'₁, but to the flour is added 5.5 g of saccharose; the total amount of gas produced is measured after 60 minutes,

Tests A₅ and A'₅

Tests identical respectively with tests A₁ and A'₁, with the difference that there is added to the yeast suspension, just before the addition of the latter to the flour, an amount of 0.15 ml of a mixture constituted by 15 g of acetic acid and 80 g of lactic acid, these 0.15 ml being substituted for 0.15 ml of dilution water,

Test B₁ (fresh compressed yeast and instant dry yeasts not needing prior rehydration)

To 250 g of flour, is added a weight of compressed yeast or instant dry yeast corresponding to 1.6 g of yeast dry material, and 150 ml of salted water (50 g of salt/1.51 of water); it is kneaded for 6 minutes; the temperature of the dough must be 27° C. at the end of kneading; the dough is placed in the apparatus and 6 minutes, measured exactly, after the end of kneading the chamber thermostatted to 27° C. is placed under pressure; the total release recorded on the graph, in ml, is measured after 1 hour and 3 hours,

Test B'₁ (dry yeasts requiring rehydration)

Tests identical with Test B₁ but prior to kneading, the dry yeast is rehydrated in distilled water at 38° C. (50 ml) for 15 minutes; the complement of water and of salt is added at the end of the 15 minutes of rehydration,

Test B'₃

Test identical with test B'₁, with the difference that there is added to the yeast suspension obtained after dilution of the fresh yeast or after rehydration of the dry yeast, just before kneading, an amount of 2 ml of a mixture constituted by 15 g of acetic acid and 80 g of lactic acid, these 2 ml substituted for 2 ml of diluting water.

The novel strains obtained can hence be characterised by the commercial yeasts whose production they permit and which will be defined below.

1. Strains giving fresh yeasts active with sweetened doughs and characterised by the fact that they give rise to:

a gas release equal to or greater than 112 and, preferably, to 115 ml of CO₂ in test A₁ in 2 hours, a gas release equal to or greater than 1500 ml of CO₂ in test B₁ in three hours and, preferably, equal to or greater than 135 ml of CO₂ in test A₁ and 1700 ml in test B₁,

a gas release equal to or greater than 53, preferably to 55 ml of CO₂ in 1 hour in test A₂ and, more preferably again, equal to or greater than 60 ml in this test in 1 hour,

a gas release equal to or greater than 50 ml of CO₂ in 1 hour in test A₃ and, preferably, equal to or greater than 55 ml in test A₃ in 1 hour,

a gas release equal to or greater than 25 ml of CO₂ in test A₄ and, preferably, equal to or greater than 30 ml of CO₂ in 1 hour and more preferably again, equal to or greater than 35 ml of CO₂, the yeast reaching the preferred values for two of the abovesaid tests being particularly preferred.

2. Strains giving dry yeast active with sweetened doughs and characterised by the fact that they give rise to:

a gas release equal to or greater than 98, preferably to 100 ml of CO₂ in test A'₁ in 2 hours, greater than or equal to 1350 CO₂ in test B₁ in 3 hours and, preferably, equal to or greater than 115 of CO₂ in test A'₁ and to 1500 ml of CO₂ in test B₁ in 3 hours,

a gas release equal to or greater than 46, preferably to 48 ml of CO₂ in test A'₂ in 1 hour, and, preferably, equal to or greater than 52 ml in test A'₂,

a gas release equal to or greater than 44 ml of CO₂ in test A'₃ and, preferably, equal to or greater than 47 ml of CO₂ in test A'₃,

a gas release equal to or greater than 21 ml of CO₂ in test A'₄ and, preferably, equal to or greater than 26 ml of CO₂.

3. Strains giving fresh yeast active with acid doughs and characterised by the fact that they give rise to:

gas release equal to or greater than 115 ml of CO₂ in test A₁ in 2 hours and equal to or greater than 1500 ml of CO₂ in test B₁ in 3 hours and, preferably, equal to or greater than 135 ml of CO₂ in test A₁ and to 1700 ml of CO₂ in test B₁,

a gas release equal to or greater than 40 ml of CO₂ in test A₅ in 1 hour and, preferably, equal to or greater than 45 ml of CO₂ in test A₅ in 1 hour, a gas release equal to or greater than 900 ml of CO₂ in test B'₃ in three hours, and preferably, equal to or greater than 1000 ml of CO₂ in this test B'₃ in 3 hours.

4. Strains giving dry yeasts active with acid doughs and characterised by the fact that they give rise to:

a gas release equal to or greater than 100 ml of CO₂ in test A'₁ in 2 hours, greater than 1350 ml of CO₂ in test B₁ in 3 hours and, preferably, equal to or greater than 115 of CO₂ in test A'₁ and to 1500 ml in test B₁ in three hours,

a gas release greater than or equal to 32 ml of CO₂ in test A'₅ in 1 hour and, preferably, equal to or greater than 38 ml of CO₂ in test A'₅ ; a gas release equal to or greater than 750 ml of CO₂ in test B'₃ in 3 hours and, preferably, equal to or greater than 820 ml of CO₂ in test B'₃.

5. Strains giving fresh yeast, on the one hand, and dry yeast, on the other hand, active with sweetened doughs and with acid doughs and characterised by the fact that

the first five rise simultaneously to gas releases shown by the abovementioned fresh yeast, active with sweetened doughs and by the abovesaid fresh yeasts active with acid doughs and

the second give rise simultaneously to the releases shown by the abovesaid dry yeasts active with sweetened doughs and by the abovesaid dry yeasts active with acid doughs.

In the foregoing, there has generally been denoted by fresh yeasts, those which have a content of dry material of about 28 to 35% and by dry yeasts those which have a content of dry material higher than 92%.

The nitrogen content of these yeasts has generally been selected within the following ranges:

about 7.5 to 8.5 for fresh compressed yeast,

about 7.2 to about 8.2 for dry yeast.

Once the strain sought is available, it is possible to prepare the fresh compressed yeast corresponding, having a content of dry matter in the neighborhood of 28 to 35% by resorting to a conventional propagation scheme adapted to provide fresh yeast stable in preservation and stable on drying. These yeasts can then be dried to about 92% of dry material or more by means of a particularly gentle drying process.

Preferably, the cultivation of the yeast is conducted so as to obtain a fresh yeast with 28-35% of dried matter having the following characteristics:

an amount of budding below 5% and, preferably, less than 1%,

cryoscopic lowering of the water external to the yeast below 0.5° C. and, preferably, below 0.3°. It is pointed out that, to measure the cryoscopic lowering of the external water of a fresh compressed yeast, a cream is produced with 100 g of the pressed yeast and 30 g of completely dimineralised water, this cream is centrifuged and cryoscopic lowering of the supernatant liquid obtained is measured, for example, by means of a Beckman type cryoscope (Prolabo no. 0329 600). The lowering of the freezing point measured is proportional to the amount of gram-molecules of dissolved substances in the external water.

If it is desired to obtain very quick yeast, fairly high nitrogen contents are selected: 8-8.5% of nitrogen to dry matter, even a little more. If it is desired to obtain yeast having more particular characteristics as for example, stability to drying, the following characteristics are rather to be sought:

protein content corresponding to the optimum of the strain cultivated taking into account the desired characteristics; this content varies according to the strains and the desired characteristics for the yeast but for relatively quick strains it is of the order of 7.5% to 8% of nitrogen to dry matter or even less; the optimum of the nitrogen content for stability to drying may be defined as being the value above which any increase in this content no longer gives more than a slight gain in activity but, after drying, an additional loss in activity equal to or greater than this gain. This optimum depends much on the strain, on the conditions of culture, on a reference test (with sugar, without sugar). It can only be determined experimentally case by case. It is of course obvious to the specialist that all the values of nitrogen to dry matter given above are only indicative; ##EQU2##

Preferably, it will be particularly sought to provide the culture medium with the growth factors of which each strain has a need: biotin, group B vitamins etc.

To the yeast intended for drying, is preferably added a fine emulsion constituted by a suitable emulsifier, such as, for example, sorbitol esters, polyglycerol esters in the proportion of 1.5 to 2% of dry yeast matter and if necessary a thickening agent.

Yeast intended for drying, of 30-35% dry matter, is extruded through a grid of mesh width 0.5 to 3 mm, and, preferably, 0.5 to 1 mm.

It is then dried to about 92% of dry matter or more, preferably to a content of dry matter comprised between 94 and 97% by a particularly gentle drying. This can be a fast pneumatic drying, a fluidised bed drying or a combination of these two methods of drying. Preferably, the drying will be conducted so that the temperature of the yeast does not exceed 30° C. at the beginning of the drying and 40° C. at the end of drying.

To enable the invention to be better understood, there are described below, be means of some examples, the production of some strains of yeast according to the invention, the preparation of fresh yeasts and dry yeasts from these strains and the properties of the fresh yeasts and dry yeasts thus obtained.

DESCRIPTION OF PREFERRED EMBODIMENTS

EXAMPLE 1

Two quick strains, adapted to maltose, stable to drying, deposited by applicants at the N.C.Y.C. under nos. N.C.Y.C. 875 and N.C.Y.C. 876, were sporulated.

Two slow strains, very osmotolerant, that is to say, very active with sweetened doughs, deposited by applicants at the N.C.Y.C. under nos. R30 and N.C.Y.C. 877, were sporulated.

For each group of strains, ten haploids of mating type a and ten haploids of mating type α, were isolated. By mass-mating, each haploid of one group was then conjugated with all the haploids of the opposite mating type of the other group. In this way 196 hybrids were obtained.

These 196 hybrids were tested with the first, the second, and the third screening tests and the fourth screening test.

The growth curves of the first and second tests were effected in optical flasks, flasks provided with a calibrated tube enabling colorimetric reading without sampling of the medium. The YEP culture medium was composed of 1% yeast extract, 2% of peptone and 2% saccharose and it was seeded with 0.3 to 0.5 ml of a stirred pre-culture of the hybrid to be tested in a liquid medium with 1% of sugar.

The second test was practiced by adding to the culture medium of the first test a glacial acetic acid in the proportion of 0.13 and 0.14 ml, namely 0.433% and 0.466%.

The third and the fourth test were carried out on the yeast harvested by centrifugation or filtration of the culture practised in the scope of the first test. The yeast harvested was washed to remove the amount of sugar from the culture medium, which sugar could falsify the results of the third test.

The third test was carried out in an 0.01 M phosphate buffer medium, pH 6.5, the reaction mixture comprising:

    ______________________________________                                         Yeast           20 to 25 mg of dry matter                                      Glucose         4 mg                                                           Maltose         2 mg.                                                          ______________________________________                                    

The reaction is carried out at 30° C. for one hour and it is arrested by sudden cooling and cold centrifugation. The determination of the supernatant sugar is done by the anthrone colorimetric method.

The fourth test is practised on 0.1 to 0.4 mg of dry yeast matter which are placed in the presence of saccharose at a final concentration of 0.1 Molar, in a test tube in a buffered medium with acetate buffer at pH 4.7, placed in a water bath at 30° C. At the end of 5 minutes, the saccharose inversion reaction is blocked by the addition of the reactant with sodium dinitrosalicylate which serves to determine reducing sugars formed by colorimetric reaction.

Within the scope of the first screening test, can be determined the coefficient μ: increase in population per unit time and per unit mass of population for quick commercial yeast strains or starting strains, the control value found being denoted by μ_(t).

Within the scope of the first test, there are eliminated all hybrids for which:

    μ<0.9μ.sub.t.

Within the scope of the second test, it is observed that all the quick strains, adapted to maltose employed have a:

    μ in 0.433% acetic acid<0.4μ.sub.t and a

    μ in 0.466% acetic acid<0.25μ.sub.t

but that a slow strain, very osmotolerant and rather insensitive to acetic acid like the strain N.C.Y.C. R30, has a

    μ in 0.433% acetic acid>0.6.sub.t and a

    μ in 0.466% acetic acid>0.4μ.sub.t.

Within the scope of this second test, will be considered as kept for subsequent examination are all strains giving:

    μ in 0.433% acetic acid>0.55μ.sub.t and a

    μ in 0.466% acetic acid>0.32μ.sub.t.

Within the scope of the third test, it is observed that quick strains considered as adapted to maltose, consume 60-80% of the maltose present, but that, on the other hand, a slow strain like N.C.Y.C. R.30 consumes less than 20% of the maltose present. Within the scope of this test, there are eliminated all strains consuming less than 35% of the maltose present and the strains consuming more than 60% of the maltose are retained. The strains consuming between 35% and 60% of the maltose present are only retained if they have been retained according to the criteria of the second test or according to the criteria of the fourth test.

In the fourth test, all the strains are kept which titrate less than 35 invertase units and, preferably, less than 30 invertase units and, more preferably, less than 20 invertase units.

Within the scope of the selection as described above, 10 hybrids of the 196 obtained by crossing were kept.

It is remarkable that the hybrids selected having the following properties:

growth curve equivalent to the controls in the first test,

adaptation to maltose characterised by a consumption of at least 35% of the maltose present in the second test, that is to say at least 50% of the maltose consumed by the control quick strains adapted to maltose,

invertase content less than 35 units and, preferably, less than 30 units and, more preferably, less than 20 units in the fourth test, or/preferably and growth in the presence of acetic acid characterised by a:

    μ in 0.433% of acetic acid>0.55μ.sub.t and/or a

    μ in 0.466% of acetic acid>0.32μ.sub.t,

that is to say lesser inhibition in the presence of acetic acid than the control quick strains adapted to maltose,

are novel strains for the production of yeast for breadmaking, no strain used in the breadmaking yeast industry combining these three or four characteristics.

These ten selected hybrids are then tested by conventional means:

cultivation in fermenters of three liters such as described in Yeast Technology, J. White (1954) pages 103 to 106 where the culture medium has a total volume of 1100 ml, the sugar is added in the form of molasses, the air is filtered through a diaphragm of the Millipore type at the rate of 1 m³ /hour and seeding is carried out by 300 mg of yeast obtained by anerobic cultivation in flasks,

culture aerated and stirred on a battery of the New Brunswick Scientific Company of 5 liters of useful volume 3/3.5 liters, then drying the yeast harvested,

or culture in a battery of pilot fermenters of 80 liters useful volume such as described in Example 2 of French Pat. No. 75 20943, then drying.

The cultures at the stage of the White type 3 liter fermenter established that 5 of the 10 hybrids selected have most interesting novel characters. In Table 1 are given the results obtained in the same series of experiments for these 5 hybrids, several hybrids of quick yeast adapted to maltose, taken as controls, and a slow strain, namely N.C.Y.C. R30 which is particularly active.

                  TABLE 1                                                          ______________________________________                                         Strains Invertase                                                                               Test A.sub.1                                                  used    units    1 hour   Test A.sub.2                                                                          Test A.sub.3                                                                          Test A.sub.4                           ______________________________________                                         Controls                                                                       (hybrius of                                                                    quick yeast                                                                    adapted to                                                                     maltose)                                                                               45 to 140                                                                               57       64     46     8 to 16                                N.C.Y.C.                                                                               25       29       46     56     26                                     R30                                                                            Hybrid 1                                                                               23       52       61     55     24                                     Hybrid 2                                                                               37       58       68     51     17                                     Hybrid 3                                                                               35       65       72     56     18                                     Hybrid 4                                                                               32       56       67     62     23                                     Hybrid 5                                                                               29       52       62     60     25                                     ______________________________________                                    

After tests in batteries of fermenters of greater volume and after tests in factories, it was hybrids 3 and 5 which were kept and filed at N.C.Y.C.

Hybrid no. 3 received the no. N.C.Y.C. 848 and hybrid no. 5 received no. N.C.Y.C. 847.

In Tables 2 and 3 are given the results obtained by these two hybrids in the factory after culture of 100m³ leading to a harvest of about 25 tons of fresh yeast, conducted so as to obtain:

a nitrogen content in the dry material of about 8%,

a P₂ O₅ content in the dry material of about 2.3%,

a trehalose content in the dry material of about 13%,

a ratio of buds of the order of 1%,

a cryoscopic lowering of the external water of the yeast of the order of 0.3° C.

By way of comparison there are given the results obtained with a hybrid yeast adapted to maltose and the N.C.Y.C. R30 (table II as regards fresh yeast and table III as regards the dry yeast).

                  TABLE II                                                         ______________________________________                                         FRESH YEASTS                                                                                      B.sub.1 B'.sub.3                                            Strain                                                                               A.sub.1    A.sub.2                                                                              A.sub.3                                                                            A.sub.4                                                                            A.sub.5                                                                            1 h  3 h  1 h  3 h                          ______________________________________                                         Quick                                                                          yeast                                                                          hybrid                                                                               55+80=135  59    49  22  28  350  1700  60   400                         adap-                                                                          ted to                                                                         malt-                                                                          ose                                                                            NCYC  37+48=85   52    56  40  33  260  1200 120   650                         R 30                                                                           NCYC  60+80=140  64    53  33  53  420  1780 200  1160                         848                                                                            NCYC  55+75=130  63    57  37  50  400  1700 250  1200                         847                                                                            ______________________________________                                    

                                      TABLE III                                    __________________________________________________________________________            DRY YEASTS                                                                                         B.sub.1                                                                              B'.sub.3                                      Strain A'.sub.1                                                                               A'.sub.2                                                                          A'.sub.3                                                                          A'.sub.4                                                                          A'.sub.5                                                                          1 h                                                                               3 h                                                                               1 h                                                                               3 h                                        __________________________________________________________________________     Quick yeast                                                                    hybrid adap-                                                                   ted to malt-                                                                          48 + 70 = 118                                                                          50 41 18 24 300                                                                               1500                                                                               45                                                                               300                                        ose                                                                            NCYC R30                                                                              32 + 43 = 75                                                                           45 47 32 29.5                                                                              230                                                                               1050                                                                               90                                                                               500                                        NCYC 848                                                                              51 + 68 = 119                                                                          53 45 25 43 350                                                                               1550                                                                              120                                                                               830                                        NCYC 847                                                                              46 + 61 = 107                                                                          53 48 28 38 300                                                                               1470                                                                              110                                                                               830                                        __________________________________________________________________________

The principal taxonomic characteristics of the strains of Saccharomyces Cerevisiae N.C.Y.C. 847 and N.C.Y.C. 848 are shown in Table VI.

EXAMPLE 2

To try to obtain strains with a low invertase content, the haploids of slow strains obtained within the scope of Example 1 which seemed most interesting, were crossed with haploids of quick strains adapted to maltose which had undergone a mutation treatment intended to lower their invertase content. The mutagenic treatment was conducted in the following manner:

the haploid strain derived from a fresh preculture was reinjected into the culture medium and brought to the exponential phase of growth. The yeast was harvested sterilely by centrifugation and resuspended in Tris buffer of pH 6.6. The final concentration of cells was 10⁷ to 10⁸ cells per ml. The nitrosoguanidine mutagenic agent was added to a final concentration of 200 to 400 micrograms/ml.

The reaction was carried out at 30° C. for a period of 15 to 60 minutes. At the end of the reaction, after dilution with a large amount of salt water, it was centrifuged cold. The treated haploid cells were then brought to a suitable dilution and spread over a gelose medium in a petri dish. According to the treated haploid strain, survival rates of 10 to 80% were obtained.

The haploids treated in the treatment which gave a survival rate higher than 40% were retained for screening.

As the screening test, to select these haploids, the first test and the fourth test were employed: invertase content less than 20 units.

This work was carried out until the production of:

5 mutant haploids of mating type a,

5 mutant haploids of mating type α responding positively to these two tests and for which it has been checked by means of the third test that their adaptation to maltose is retained.

These hybrids were crossed with:

6 haploids derived from slow strains of mating type a,

4 haploids derived from slow strains of mating type alpha.

In this way 50 hybrids were obtained which were tested by means of the first test, the fourth test (invertase content less than 20 units) and the third test as described in Example 1.

7 hybrids were selected by means of these screening tests.

After White cultivation, tests in New Brunswick type fermenters and tests on the factory scale, 1 hybrid was kept. This hybrid was deposited at the N.C.Y.C. under no. N.C.Y.C. 878. The results obtained with this hybrid in test fermenters of the WHITE type of 3 liters, and in factory tests are reported in Table IV.

                  TABLE IV                                                         ______________________________________                                                    Test A                                                                       Invert-         Total Test Test Test Test                                      ase     1 hour  2 hours                                                                              A.sub.2                                                                             A.sub.3                                                                             A.sub.4                                                                             A.sub.5                          ______________________________________                                         Results of                                                                     yeasts with                                                                    32% dry                                                                        matter after                                                                   cultivation in                                                                 WHITE type                                                                     fermenters.                                                                             15      43            53   61   28                                    Results of                                                                     fresh yeasts                                                                   after factory                                                                  trials.   5      42+70   112   53   61   44   35                               ______________________________________                                                      Test A'.sub.1                                                                              A'.sub.2                                                                              A'.sub.3                                                                            A'.sub.4                                                                            A'.sub.5                             ______________________________________                                         Results of dry yeasts                                                          after factory trials                                                                        37+62=99    47     48   35   31                                   ______________________________________                                    

EXAMPLE 3

The most interesting strains obtained in examples 1 and 2 were made to sporulate.

The haploids obtained from these novel strains of yeast were crossed with the haploids which were most interesting, that is to say those which led once or, preferably, several times to selected strains.

In this way 110 crossings were carried out which led to 21 hybrids retained after selection according to the first test, the fourth test and the third test, all the three applied as in Example 1.

After trials, 2 hybrids were finally retained and deposited at the N.C.Y.C. under nos. N.C.Y.C. 879 and N.C.Y.C. 880.

The results obtained with these two hybrids in White type fermenters of 3 liters in tests are reported in table V below. Their taxonomic characteristics are reported in table VI. The 5 hybrids obtained within the scope of these examples were all identified as Saccharomyces Cerevisiae, by the NCYC, where the hybrids are deposited.

                  TABLE V                                                          ______________________________________                                         Results with cult-    Test A.sub.1                                             ure in WHITE type     in 1      Test Test Test                                 fermenters   Invertase                                                                               hour      A.sub.2                                                                             A.sub.3                                                                             A.sub.4                              ______________________________________                                         N.C.Y.C. 879 21       48        58   63   28                                   N.C.Y.C. 880 16       42        54   64   31                                   ______________________________________                                    

These two strains N.C.Y.C. 879 and 880 have properties close to those of strain N.C.Y.C. 878, that is to say they are quicker in all the tests than N.C.Y.C. R30 including Test A₄ corresponding to a very sweet cough.

EXAMPLE 4

Throughout the work described in Examples 1 to 3, it became recognisable that a certain number of haploids were of particular interest leading to selected hybrids and recognised as having interesting novel properties.

5 of these particularly active haploids were deposited at the N.C.Y.C. These were the haploids:

Ha 1 haploid of mating type a which has received no. N.C.Y.C. 881

Ha 2 haploid of mating type a which has received no. N.C.Y.C. 882

Hα3 haploid of mating type α which has received no. N.C.Y.C. 883

Hα4 haploid of mating type α which received no. N.C.Y.C. 884

Hα5 haploid of mating type α which has received no. N.C.Y.C. 885

These haploids were all obtained by sporulation of Saccharomyces Cerevisiae strains.

These haploids cultivated in White type fermenters gave the following results:

    ______________________________________                                         Inver-                                                                          contenttase                                                                             in 1 hourTest A.sub.1                                                                   in 1 hourTest A.sub.4                                                                   ##STR1##                                           ______________________________________                                         Ha.sub.1                                                                            18      41       29     121                                               Ha.sub.2                                                                            11      29       17     109                                               Hα.sub.3                                                                      85      50       13     75                                                Hα.sub.4                                                                              33       15     89                                                Hα.sub.5                                                                      13      26       35     139                                               ______________________________________                                    

The two first haploids Ha₁ (N.C.Y.C. no. 881) and Ha₂ (N.C.Y.C. no. 882) were particularly remarkable:

since they have quick characters, that is to say adaptation to maltose, and osmotolerant, that is to say adaptation to high sugar content,

since they impose the low invertase characteristic after conjugation, that is to say they give systematically diploid strains with low invertase.

They gave interesting results when they were conjugated with haploids derived from quick yeasts as well as when they were conjugated with haploids derived from slow yeasts.

The two haploids Hα₃ (N.C.Y.C. no. 883) and Hα₄ (N.C.Y.C. no. 884) are interesting and act as examples of haploids derived from non-osmotolerant quick yeasts.

Haploid Hα₅ is a good example of a haploid having characters of very good adaptation to sweetened doughs; haploid Hα₅ has N.C.Y.C. no. 885.

EXAMPLE 5

Following the work described in examples 1 to 4, the 5 selected strains are made to sporulate, and the haploids obtained from these 5 strains are then crossed with the most performing haploids (i.e. the haploids having led most often to selected hybrids) derived from quick yeast well adapted to maltose and themselves presenting those characteristics in order to obtain new yeast hybrids quicker and even better adapted to maltose than those obtained up till now. The main screening tests retained for this project were the first and third, the other screening tests, and particularly the fourth, being carried out only on the most performing hybrids according to the first and third screening tests.

This hybridization and selection work led to the selection of the hybrid noted in our registers under the number of a α1217.

This newly-obtained strain corresponding to the diploid strain obtained by mass-mating was first sent to the National Collection of Yeast Cultures (NYCY) the present address of which is: Agricultural Research Council, Food Research Institute, Colney Lane, Norwich NR4 7UA, and was given the deposit number NCYC 889.

This new strain corresponding to a purified isolation of the strain obtained directly by mass-mating, was later sent a second time to the NCYC and given the deposit number NCYC 890.

Since the 2 parent strains belong to the species Saccharomyces Cerevisiae, this new strain a α1217 also belongs to this same species.

The following results from the different trials carried out on the new strain a α1217 were obtained:

(1) Results obtained from the fresh yeast cultured to White-type-fermenters:

    ______________________________________                                                  In-           Test    Test  Test                                               ver- Test A.sub.1                                                                            A.sub.2 A.sub.3                                                                              A.sub.4                                            tase in 1 hr. in 1 hr.                                                                               in 1 hr.                                                                             in 1 hr.                                  ______________________________________                                         NCYC 889                                                                               1st    19,5   52,6   60,4  55,8  22,1                                  strain ob-                                                                             trial                                                                  tained after                                                                   mass-                                                                          mating                                                                                 2nd           49,1   58,2  52,9  21                                            trial                                                                  Purified              52,6   61    56    23,5                                  strain                                                                         NCYC 890                                                                       ______________________________________                                    

(2) Results obtained from the fresh yeast cultured in the NBS pilot fermenters (New Brunswick Scientific Co. Inc.):

    ______________________________________                                         Nitrogen on                                                                              Test A.sub.1                                                                             Test A.sub.2                                                                             Test A.sub.3                                                                          Test A.sub.4                              dry matter                                                                               in 2 hours                                                                               in 1 hr.  in 1 hr.                                                                              in 1 hr.                                  ______________________________________                                         7,2       134       63        53     24                                        ______________________________________                                    

(3) Results from the fresh yeast intended for drying and obtained after factory tests with a culture of 100 m³, giving approximately 25 tons of fresh yeast (non-commercialised yeast):

    ______________________________________                                         Nitro-                          Test Test Test                                 gen/                            A.sub.2                                                                             A.sub.3                                                                             A.sub.4                              on dry   Inver-  Test A.sub.1   in 1 in 1 in 1                                 matter   tase    in 1 hr + 1 hr = 2 hrs                                                                        hour hour hour                                 ______________________________________                                         Trial                                                                               8,6     47      63 + 86 = 149                                                                               67   60   32                                 Trial                                                                               8,4     43      65 + 88 = 154                                                                               67   58   33                                 Trial                                                                               7,9     40      60 + 83 = 143                                                                               63   57   36                                 Trial                                                                               7,5     42      58 + 81 = 139                                                                               62   57   34                                 Trial                                                                               7,6     40      57 + 80 = 137                                                                               62   55   32                                 ______________________________________                                          (4) Results from the dried yeast after drying between 95 and 96% of dry      matter from the yeast obtained in the mentioned factory tests:

    ______________________________________                                                         Test    Test    Test                                                           A'.sub.2                                                                               A'.sub.3                                                                               A'.sub.4                                                                             Test B.sub.1                             Test A'.sub.1   in 1    in 1    in 1  in 3                                     in 1hr + 1hr = 2hrs                                                                            hour    hour    hour  hours                                    ______________________________________                                         Trial 1.                                                                             53 + 71 = 124 57      49    27                                           Trial 2.                                                                             56 + 74 = 130 58      50    27                                           Trial 3.                                                                             52 + 70 = 122 53      48    29    1580                                   Trial 4.                                                                             53 + 70 = 123 56      49    26    1670                                   Trial 5.                                                                             52 + 69 = 121 53      47    26                                           ______________________________________                                    

These results show:

that the yeasts obtained from this strain are as quick and as well adapted to maltose as those yeasts obtained with the best hybrids of quick yeast adapted to maltose, and that in addition, they perform on sweet doughs as well as those obtained with a strain such as the yeast strain NCYC R 30,

that the yeasts obtained fromt the strain a α1217 are particularly stable in drying,

that the active dry yeasts obtained from this strain in factory tests are particularly performing since:

in the test A'₁, there is a gas release in the first hour amounting to between 45 and 58 ml, and in the 2 hours, a release of between 115 to 130 ml of CO₂, and preferably in 1 hour 47 to 58 ml and in 2 hours, 118 to 130 ml, but more preferably 50 to 58 ml in 1 hour and 120 to 130 ml of CO₂ in 2 hours,

in the test B'₁, there is a gas release in 3 hours between 1400 and 1800 ml, and preferably between 1450 and 1800 ml, and more preferably between 1500 and 1800 ml,

in test A'₂, there is a gas release in 1 hour between 50 and 60 ml, but preferably between 52 and 60 ml,

in test A'₃, there is a gas release in 1 hour between 45 and 54 ml, but preferably between 47 and 54 ml,

in test A'₄, there is a gas release in 1 hour between 23 and 30 ml, but preferably between 26 and 30 ml.

This new yeast strain a α1217 has a weak performance and acid doughs in tests A'₅ and B₃, performs may be little better than the quick yeast test hybrids adapted to maltose and is much less active than strains such as NCYC R 30.

EXAMPLE 6

Following the research and discoveries described in the previous Examples 1 through 5 herein, further investigations were conducted for the development of further improved yeast strains. As starting materials, many Saccharomyces diploid strains, available from public collection centers such as N.C.Y.C. (National Collection of Yeast Cultures) or as had been previously obtained from the foregoing Examples were used. In this continuing research investigation, there was involved certain crossings, made according to the processes described by R. R. Fowell. In these crossings, certain segregants of, i.e., haploid strains, providing characteristics of quick maltose fermentation and resulting from the man-induced sporulation of previously obtained man-made hybrids, are crossed with other segregants which provided characteristics of osmotolerance, and with a low invertase content, and issuing either from isolates or man-made hybrids.

By use of the above described screening tests, and in particular the first, third and fourth screening tests, and the results therefrom, a selection of novel man-made yeast hybrids was achieved.

At this point, the work had led to the creation of several thousand man-made yeast hybrids. There was then a selection made of several of these man-made hybrids having properties equivalent to the man-made hybrids already selected in the above-given foregoing Examples, this selection also being made by means of the above-described screening tests and the results of cultures in White-type fermenters, and afterwards the results of cultures obtained in NBS pilot fermenters, and drying of the same.

These thus-selected and prepared novel man-made hybrids were then compared with the man-made hybrids selected and prepared in the foregoing Examples hereinabove, especially with respect to the following characteristics:

(a) The yield on cultivation with beet molasses, i.e., the number of kilograms of yeast dry matter produced from 100 kg of molasses, according to a conventional cultivation process, using always molasses obtained from the same batch.

(b) Fermentative activities of the active dry baker's yeast composition, containing an emulsifying agent and having about 95% of yeast dry matter, such fermentative activities being those which can be obtained with each strain in the series of the above-described A' tests, and after rehydrating the dry yeast at 38+ C. as is normally done and also after such rehydration at 20° C.

These comparisons, and particularly the criterion of the yield on molasses, permitted the selection of two particular novel man-made hybrids which have now been deposited under the form of biologically pure yeast strains at the National Collection of Yeast Cultures, Agricultural Research Council, Food Research Institute, Colney Lane, Norwich NR4 7UA, under the N.C.Y.C. numbers:

N.C.Y.C. 995

N.C.Y.C. 996.

These two man-made yeast hybrids are also noted in applicant's laboratory registers under the numbers a α-5900 (for N.C.Y.C. 995) and a α-1112 (for N.C.Y.C. 996). Identification tests carried out on these strains by the N.C.Y.C. show that these strains belonging to the Saccharomyces Cerevisiae classification. The Table VI describes some of these identification tests.

Further comparisons have been made on a dry yeast basis as follows. The new strain N.C.Y.C. 995 is very comparable with the strain N.C.Y.C. 890 on a dry yeast basis. However, the strain N.C.Y.C. 995 has the advantage that it exhibits an increase of about 3% in yield on beet molasses with 50% of the total sugars expressed in saccharose, i.e., for the same amount of molasses, there will be produced 3% more of yeast dry matter. A similar increase in yield on molasses is also observed when this strain is compared to strain N.C.Y.C. 848. The strain N.C.Y.C. 995 also permits the preparation of fresh yeasts, having between 28% and 35% of dry matter, which are both quicker and better adapted to maltose, with the same amount of nitrogen as that of strain N.C.Y.C. 890; but, on the other hand, strain N.C.Y.C. 995 is very slightly less stable upon drying as compared to strain N.C.Y.C. 890. However, the amount of nitrogen which corresponds to the optimum composition of the yeast intended for drying and as obtained with strain N.C.Y.C. 995, i.e., the nitrogen content for the composition giving the most active dry yeasts, is about 7.5% nitrogen, calculated on dry matter. This is contrary to what is observed with strain N.C.Y.C. 890 where the amount of nitrogen should not be below 8%.

The strain N.C.Y.C. 996 on a dry yeast comparison basis, is very comparable with the above-mentioned strain N.C.Y.C. 878. However, the strain N.C.Y.C. 996 has the advantage of providing a 2-4% yield increase, as compared to the several strains N.C.Y.C. R 30, N.C.Y.C. 878 and N.C.Y.C. 879.

These two new novel strains N.C.Y.C. 995 and N.C.Y.C. 996 also exhibit the advantage of being able to form active dry yeasts which have a better tolerance towards rehydration at 20° C. in the A' tests, described above, than do the other-mentioned strains.

Accordingly, if one sums up the results obtained with the two new biologically pure man-made hybrids deposited at N.C.Y.C. under the numbers 995 and 996, the following results are achieved.

(1) Results obtained from the fresh yeast cultured in White type fermenters:

    ______________________________________                                         Invertase    Test A.sub.1                                                                            Test A.sub.2                                                                            Test A.sub.3                                                                          Test A.sub.4                             Units        1 hour   1 hour   1 hour 1 hour                                   ______________________________________                                         NCYC 995                                                                               38       71.6     75.5   53.1   16.3                                   NCYC 996                                                                               17       43       57     68     30                                     ______________________________________                                    

(2) Results obtained from the fresh yeast cultured in NBS pilot fermenters:

    ______________________________________                                                Nitrogen/                                                                               Invertase Test A.sub.1                                                                            Test A.sub.4                                       Dry Matter                                                                              Units     2 hours  1 hour                                      ______________________________________                                         NCYC 995 7.5        45        150    23                                        NCYC 996 7.4        20        104    32                                        ______________________________________                                    

(3) Results obtained from fresh yeast intended for drying and obtained after factory tests with a culture of 100 m³, giving approximately 25 tons of fresh yeast:

    ______________________________________                                                 Nitrogen/  Test A.sub.1                                                                            Test A.sub.4                                               Dry Matter 2 hours  1 hour                                             ______________________________________                                         NCYC 995  7.5          154      32                                             NCYC 996  7.2          105      41                                             ______________________________________                                    

(4) Results obtained from dried yeast after drying between 95% and 96% of dry matter from the yeast obtained in the above-mentioned factory tests:

    ______________________________________                                                      Test A'.sub.1                                                                  2 hours                                                                               Test A'.sub.4                                              ______________________________________                                         NCYC 995       131      26.3                                                   NCYC 996        91      34                                                     ______________________________________                                    

(5) Yield comparisons, based on the equation: ##EQU3## as obtained from the different biologically pure man-made yeast strains described hereinabove.

These tests have been carried out with the apparatus and under the conditions as described in Example 2 of French Pat. No. 7,520,943, which corresponds to British Pat. No. 1,539,211 and Canadian Pat. No. 1,075,777.

These tests have been replicated several times for each strain, using a batch of beet molasses having 42% sugar content. The following comparative results have been obtained, rated by assigning the number 100 to the yield from the strain giving the best result, within this series of tests (i.e., in this instance the strain N.C.Y.C. 995):

    ______________________________________                                         N.C.Y.C.        R30    92.7                                                    N.C.Y.C.        848    94.5                                                    N.C.Y.C.        878    90.6                                                    N.C.Y.C.        879    96.3                                                    N.C.Y.C.        890    96                                                      N.C.Y.C.        995    100                                                     N.C.Y.C.        996    98.9                                                    ______________________________________                                    

The yield increases resulting from the strains N.C.Y.C. 995 and N.C.Y.C. 996 have also been noticed in several tests carried out with different molasses batches.

(6) Behavior of the dry yeasts upon rehydration at 20° C., in comparison with rehydration at 38° C.:

The results obtained in rehydration at 20° C. in test A' and expressed as a percentage of the results obtained upon rehydration at 38° C. very much depend upon the drying conditions employed. However, the many tests which have been carried out show that the divergence between the two results is generally two times smaller with the two strains N.C.Y.C. 995 and N.C.Y.C. 996. Under rapid and gentle drying conditions, where the yeast is finely divided, with a yeast temperature never exceeding 35° C., identical results can be obtained with these two strains in the series of tests A' upon rehydration at 38° C. or rehydration at 20° C.

The results on the whole show that:

(a) From the man-made yeast strain N.C.Y.C. 995, there can be obtained fresh yeasts containing between 28 and 35% of dry matter which have a very high activity, over 150 ml of CO₂ in test A₁, while having relatively low nitrogen contents based on their dry matter content.

(b) From man-made yeast strain N.C.Y.C. 995, there can also be obtained an active dry baker's yeast composition, in solid and porous particulate form, consisting essentially of an active dry baker's yeast having an emulsifying agent therewith, said composition having a dry matter content of between about 94% and 97% and being characterized by the fact that conjointly:

in test A'₁, in 2 hours, it gives a gas release of 125 to 135 ml of CO₂, if the 160 mg essay dose of dry matter of said dry yeast is rehydrated with 6 ml of distilled water at 38° C., and also if the 160 mg essay dose of dry matter of said dry yeast is rehydrated with 6 ml of distilled water at 20° C.;

in test A'₄, in 1 hour, it gives a gas release between 23 and 28 ml, if dry yeast is rehydrated at 38° C. and also at 20° C.

(c) From man-made yeast strain N.C.Y.C. 996, there can be obtained an active dry baker's yeast composition, in solid and porous particulate form, consisting essentially of an active dry baker's yeast and having an emulsifying agent therewith, said composition having a dry matter content of between about 94% and 97% and being further characterized by the fact that it gives in test A'₄ a gas release of between 30 ml and 37 ml, when the dry yeast is rehydrated at either 38° C. or at 20° C.

(d) The biologically pure man-made hybrids of yeast strains N.C.Y.C. 995 and N.C.Y.C. 996 permit the obtention of yields of yeast dry matter which are superior by several percent to the yields obtained from the biologically pure man-made yeast hybrids obtained within the previously described research work, hereinabove.

                                      TABLE VI                                     __________________________________________________________________________     Results of identification tests carried out by the NCYC (National              Collection of Yeast Cultures)                                                  on the deposited strains                                                       Indication of some particular results on each strain                                                             Galac-                  Growth                           Size of cells in microns                                                                             tose                    in                                         Solid Number of                                                                            ter-                                                                               Assimilation        medium               Strains                                                                            Identification                                                                         Liquid medium                                                                            medium                                                                               ascospores                                                                           men-                                                                               Tre-                                                                               Melezi-                                                                            Inu-                                                                              Eryth-                                                                             αMethyl-                                                                      without              tested                                                                             result  24 h 72 h 72 h  per ascus                                                                            tation                                                                             halose                                                                             tose                                                                               lin                                                                               ritol                                                                              glucoside                                                                           vitamines            __________________________________________________________________________     NCYC                                                                               Saccharomyces                                                                          (3.5-5) ×                                                                     (2-5) ×                                                                       (2-4) ×                                                                        1 to 4                                                                               +   +   +   +  -   +    -                    847 Cerevisiae                                                                             (6.5-9)                                                                             (3.5-7.5)                                                                           (3-7)           latent                                   NCYC                                                                               Saccharomyces                                                                          (2.5-                                                                               (3-4.5) ×                                                                     (2.5-4.5) ×                                                                    1 to 4                                                                               +   +   +   +  -   +    weakly +             848 Cerevisiae                                                                             4.5) ×                                                                        (4-7)                                                                               (3-8)                                                                (4.5-6.5)                                                          NCYC                                                                               Saccharomyces                                                                          (3-5) ×                                                                       (2.5-5) ×                                                                     (2-4) ×                                                                        1 to 4                                                                               +   +   +   +  -   +    -                    878 Cerevisiae                                                                             (5-8)                                                                               (4-6)                                                                               (4-8)                                                    NCYC                                                                               Saccharomyces                                                                          (3-5.5) ×                                                                     (2-6) ×                                                                       (3-5) ×                                                                        1 to 4                                                                               +   +   +   +  -   +    ±                 879 Cerevisiae                                                                             (4-7)                                                                               (3-8.5)                                                                             (3-7)                                                    NCYC                                                                               Saccharomyces           2 to 4                                                                               +   +   +   +  -   +    ±                 880 Cerevisiae                        latent                                   NCYC                                                                               Saccharomyces                                                                               (3.6-6) ×                                                                     (1.5-5.5) ×                                                                    1 to 4                                                                               +   +   +   +  -   +    -                    875 Cerevisiae   (4-8)                                                                               (3.5-8)         latent                                                                             latent     latent                    NCYC                                                                               Saccharomyces                                                                               (3-7) ×                                                                       (3-5) ×                                                                        1 to 4                                                                               +   +   +   +  -   ± -                    876 Cerevisiae   (4-10)                                                                              (4-11)          latent                                                                             latent                               NCYC                                                                               Saccharomyces                                                                               (2-4.5) ×                                                                     (1.5-4.5) ×                                                                    2     +   +   +   +  -   +    weak                 877 Cerevisiae   (3-6)                                                                               (2.5-9)         latent                                                                             latent                               NCYC                                                                               Saccharomyces                                                                          (1.5-8) ×                                                                     (1.5-8) ×                                                                     (2.9-8) ×                                                                      1 to 2                                                                               +   +   +   -  -   +    -                    R30 Cerevisiae                                                                             (2-7)                                                                               (2-6)                                                                               (2.5-7)     3                                                                              weeks                                        NCYC                                                                               Saccharomyces                                                                               (3-5) ×                                                                       (2.5-5) ×                                                                      2     +   +   +   ± ?                                                                            -   +    ±                 890 Cerevisiae   (4-8)                                                                               (4-8)       weak                                                     after 48 hours                                                                           after 48 h                                               NCYC                                                                               Saccharomyces                                                                          (5-6) × (5-10)                                                                     (5-6) ×                                                                        2 to 3                                                                               +   +   +   -  -   +.sub.WL                                                                            -                    995 Cerevisiae        (5-10)          latent                                               after 48 hours                                                                           after 48 h                                               NCYC                                                                               Saccharomyces                                                                          (3-5) × (5-7)                                                                      (3-7.5) ×                                                                      1 to 4                                                                               +   +   +   -  -   +.sub.WL                                                                            -                    996 Cerevisiae        (4.5-5.5)                                                __________________________________________________________________________      Remark:                                                                        The twelve strains described have been characterised as belonging to the       Saccharomyces Cerevisiae species. The other characters described in this       table are secondary characters, without technological significance. Their      reproducibility within the scope of the tests practiced (J LODDER tests)       is not always ensured                                                    

The last two Examples demonstrate that the screening tests described herein lead rapidly to the obtaining of novel biologically pure man-made yeast strains which are of particular interest for the production of fresh baker's yeast or active dry baker's yeast.

Example 4 shows that the screening tests described also rapidly lead to defining particularly active haploids, which can impose the desired characteristics such as, for example, a low invertase content.

Whatever embodiment is adopted, there are thus provided:

(i) On the one hand, new and novel biologically pure man-made yeast strains,

(ii) On the other hand, fresh yeasts and active dry baker's yeast compositions obtained from these novel man-made yeast strains and constituting themselves novel industrial products, these respective novel man-made yeast strains as well as the fresh and dried yeast compositions derived therefrom having the numerous advantages explained hereinabove with respect to those strains hitherto existing.

This invention relates also to the bread baking products containing the fresh yeast and the dry yeasts produced by means of these novel man-made strains of yeast thereby obtained.

It is self-evident and will be understood by those skilled in the art that this invention is in no way limited to the specific embodiments and adaptations which have been specifically described hereinabove, but, on the contrary, encompasses all those additional modifications which fall within the spirit and scope of the following claims. 

We claim:
 1. The biologically pure man-made yeast strain deposited at the NCYC under the number
 995. 2. Fresh baker's yeast composition obtained by a cultivation process on molasses of the man-made yeast strain NCYC 995 of claim 1 and containing dry material of about 28 to 35%.
 3. Active dry baker's yeast composition obtained from man-made yeast strain NCYC 995 of claim 1 and further including an emulsifying agent.
 4. The biologically pure man-made yeast strain deposited at the NCYC under the number
 996. 5. Fresh baker's yeast composition obtained by a cultivation process on molasses of the man-made yeast strain NCYC 996 of claim 4 and containing dry material of about 28 to 35%.
 6. Active dry baker's yeast composition obtained from man-made yeast strain NCYC 996 of claim 4 and further including an emulsifying agent. 